What electrical signal does the speedo use?
 

I'd like to calibrate my speedo as I believe the needles has been moved on its post during a previous repair, so I am hoping someone can tell me what signal the Speedo receives as input and from where.
cheers
Jeff

The Speedo Signal comes from the transmission Via Cable and the signal used as far as I am aware is Magnetic. The spinning magnet creates a magnetic field depending on who fast you go...the faster the speed the higher the field...the higher the speed indicated by the needle on your Speedo. To adjust or recalibrate it requires as far as I know a magnetic recalibration machine. I´d probably contact a recalibration company for their advice as its something I personally wouldn´t feel comfortable about messing with.

Is yours an Elertronic Speedo?

The electronic speedometers use pulses from magnets in the transmission. The sending unit mounts on the outside of the transmission and has two wires that go to the speedo. Some of the speedos have an adjustable pot inside that can be used to adjust the speed, but many don't. The tachometer has no effect on the speedo. I was able to calibrate this one to work with my 3.2 conversion on my 1973.5
http://forums.pelicanparts.com/porsche-911-technical-forum/333215-1986-speedometer.html
http://forums.pelicanparts.com/uploa...1289218989.jpg

Thanks guys.

If you know your rpm & gear ratio, you can use that to calculate speed/calibrate speedometer.

Or use a GPS.

First check if your indicated speed is off from the real speed.
Use the Miles marker on the road, drive at constant speed and record the time it took you to drive.
Do some simple math and you have your speed. If you drive one mile and use a chronometer it is easy. 60 seconds in one mile is 60 miles/hour, 45 seconds in a mile is 80 miles/hour, 90seconds in a mile is 40 miles/hour. The math is simple 3600/seconds it took to drive a mile = speed in
miles /hour.
After you have your real speed then you can calibrate your speedometer, you just need to check how much is the difference between real and indicated speed.
The indicated speed can change if you change tires sizes, it will never be exact.

i would think it is pulsed signal since it is magnetic. you would have to know how many pulses per second (PPS) for a given speed. it could be anything. it could be 60 PPS for 60mph or 120PPS for 60mph.

its kinda like our radar antenna. the old one was 4096 pulse per 1 revolution, the new one is 15,000 pulses per revolution, same 4.7 second antenna speed.

send it off.

I have a good document describing how many pulses per second are generated. I put this together when I was building a special circuit to record speed on my laptop via my WBO2 controller.
In short you get 8 pulses generated per wheel rotation.

If you want the doc PM me, I'll send it.